1. Field of the Invention
The present invention relates to a method for fine patterning and particularly to an improvement in a method for fine patterning useful in manufacturing of semiconductor devices.
2. Description of the Background Art
A lift-off method is known as a method for forming a fine conductor pattern for a semiconductor device and it is described for example by J. M. Frary and P. Seese in Semiconductor International, Vol. 4, No. 12, 1981, pp. 72-88. The lift-off method is easier in dimensional control of patterns compared with an etching method and makes it possible to form a fine pattern with a high resolution.
FIGS. 1A to 1D are schematic sectional views illustrating an example of a conventional lift-off method.
Referring to FIG. 1A, a resist layer 2 is applied to a substrate 1. Light ions which can attain the substrate 1 are implanted into predetermined regions 3 of the resist layer 2 by a focused ion beam (FIB) method for example.
Referring to FIG. 1B, the resist layer 2 is developed by a suitable solvent, so that the ion-implanted regions 3 are removed. As a result, a pattern of the resist portions 2a is left on the substrate 1.
Referring to FIG. 1C, conductor layers 4a and 4b are deposited from above by vacuum evaporation for example to cover the upper surfaces of the resist portions 2a and the exposed upper surface regions of the substrate 1. After that, the resist portions 2a are removed by using a suitable solvent and consequently the conductor layers 4a on the resist portions 2a are lifted off. On this occasion, the solvent contacts the resist portions 2a through the gaps between the conductor layers 4a and 4b as shown by the arrow 1G.
Referring to FIG. 1D, a desired conductor pattern 5 is left on the substrate 1 after the resist portions 2a and the conductor layers 4a thereon have been removed.
In the above described process, if the conductor pattern 5 becomes fine to increase an aspect ratio (i.e., a ratio of a height to a width) of the conductor 4b, the respective gaps 1G between the conductor layers 4a and 4b in FIG. 1C tend to be decreased. Accordingly, insufficient contact occurs in the resist portions 2a and the solvent through the respective gaps 1G, and the conductor layers 4a on the resist portions 2a are liable to be incompletely lifted off. Thus, if conductor portions 4b of a submicron or quartermicron width are formed on the substrate 1, the resist portions 2a and the conductor layers 4a thereon might partially remain.
FIGS. 2A to 2D are schematic sectional views illustrating another lift-off method which was devised prior to the present invention.
Referring to FIG. 2A, a resist layer 21 is applied to a silicon substrate 20. Heavy ions such as gallium are implanted into predetermined regions in the surface of the resist layer 21.
Referring to FIG. 2B, the resist layer 21 is etched by oxygen plasma and a resist pattern 24 for lift-off is formed. On this occasion, since the ion-implanted regions 22 have durability to the oxygen plasma, resist portions 21a remain in regions under the implanted regions 22. However, since the resist layer 21 is isotropically etched by the oxygen plasma, undercuts 25 are also formed under the implanted regions 22.
Referring to FIG. 2C, a conductor material such as a metal is deposited from above the resist pattern 24 by vacuum evaporation, thereby forming a conductor layer 26 on the respective implanted regions 22 and a conductor layer 27 on the substrate 20.
Referring to FIG. 2D, the resist pattern 24 is removed by a suitable solvent and accordingly the conductor layers 26 on the implanted regions 22 are lifted off. As a result, a desired conductor pattern 28 is left on the substrate 20.
As can be seen from FIG. 2C, the undercuts 25 make it easy to contact the respective resist portions 21a and the solvent through the gaps 2G between the implanted regions 22 and the conductor layer 27 on the substrate 20. Consequently, the conductor layers 26 on the implanted regions 22 can be lifted off more reliably.
However, if the aspect ratio of the conductors 27 becomes large, the below described disadvantages are involved.
In order to increase the thickness H of the conductor pattern 28 as shown in FIG. 2D, it is necessary to increase the height h of the resist portions 21a as shown in FIG. 2B.
However, as shown in FIG. 3, after the resist layer 21 is etched to the depth t of the implanted regions 22, the resist layer 21 is also etched under the implanted regions 22. In this case, the etching rate is isotropic and accordingly the resist layer 21 is etched concentrically with each of the lower corners Q and P of the implanted regions 22. In consequence, even if the height h from the surface of the substrate 20 to the bottom surface of each implanted region 22 is increased, the respective implanted regions 22 are separated from the resist layer 21 when the etching depth d becomes equal to a half of the width w of each implanted region 22. Thus, the implanted regions 22 can not be supported in the predetermined positions and the structure of the resist pattern 24 for the lift-off is deformed. If the fine conductor pattern 28 is formed, the width w of each implanted region 22 becomes small. In that case, the height h of the respective resist portions 21a shown in FIG. 2B can not be made large and conductors 27 of a high aspect ratio can not be formed.